Abstract : Marmoset monkeys contracted hepatitis after having been infected with adenovirus isolated from hepatitis patients, which could be proved by enzyme values and histological findings. Re- or superinfection of the animals was carried out during their reconvalescence by injecting a long-passaged infectious monkey serum which, when administered intramuscularly, proved capable of inducing hepatitis in all infected and all unpretreated monkeys. Regardless of the fact, whether the infectious monkey serum used for challenge originated from the own laboratory or from Dr. Deinhardt, USA, nine out of 11 monkeys remained healthy. The significance of the presented experimental results with a view to the etiological elucidation of human virus hepatitis is being discussed.